1. The Field of the Invention
The present invention relates to the use of magnetites and magnetic separation to bind and remove heavy metals from water. More specifically, the present invention relates to binding heavy metals in water to magnetite and then removing those bound heavy metals from the water by magnetic separation. In one aspect of the invention the heavy metal-containing magnetite is removed from the water by flowing the water past a solid matrix displaying remnant magnetism.
2. The Relevant Technology
Water pollution is a serious problem in the United States and throughout the world. In the last several decades water pollution has been the subject of increased government scrutiny and regulation. In response to the need for clean drinking water and the need to maintain clean water in rivers, lakes, oceans, and wet lands, detailed statutory and regulatory schemes have been developed at the state and local levels in the United States. These statutory and regulatory schemes relate to many types of activities that can produce water pollution. Such activities include everything from controlling the quality of run off from farming operations and in storm drainage systems, to the regulation of industrial, mining, and commercial activities. Almost any activity that produces or has the capability of producing significant quantities of tainted water will be regulated by state and federal regulations. Several regulatory agencies deal extensively with the regulation of water emissions. Some of these agencies include the United States Environmental Protection Agency (EPA) which has broad regulatory authority, the United States Coast Guard which regulates the use of navigable waters, state Oil and Gas Boards which regulate produced waters at well sites, and state and federal agencies dealing with mine regulation.
Various water pollutants, and specific activities which have the capability of producing water pollution, are the subject of water quality regulations. Materials that may be regulated range from pesticides and fertilizers, to oil and numerous chemicals and hazardous materials. Any material that may arguably be detrimental if placed in the water supplies of the nation are subject to regulation and controlled emission.
Heavy metals are one class of problematic water pollutant which is encountered widely in the environment. Essentially all transition metals can exist as dissolved ions in water. Examples of heavy metals which may become water pollutants include lead, manganese, cobalt, cadmium, and others. These materials present significant water pollution problems when they exist in a stream or water supply. Consuming water containing heavy metals is detrimental to the health of humans and animals alike. Heavy metal poisoning can be a serious public health issue. Accordingly, there is significant interest in removing heavy metals from water supplies.
At the same time, removal of heavy metals from water in bulk is a difficult and expensive process. While the chemistry of heavy metals is well understood, applying that chemistry to remove heavy metals from water in the environment and at ambient conditions has proven difficult and expensive. These processes often require large bulky processing facilities and can produce waste products which are themselves hazardous and pose difficult disposal issues.
Adding to the problem is that fact that some old industrial and mining operations have produced heavy metal emissions over many decades and in some cases for more than a century. Many of these operations pre-date modem water pollution control regulation and the development of modem water pollution control technology. Thus, these operations produced heavy metal emissions with not much effort directed to removal of the metals from the water or limitation of the pollutants prior to their release into the environment.
Where these types of facilities have continued in operation, they have been brought up to standards by the application of the necessary modifications as required by the regulatory schemes mentioned above. However, in cases where mines and other industrial facilities closed down prior to the implementation of pollution control systems and technology and the implementation of modem regulatory schemes, it is quite possible for such facilities to continue to produce water emissions tainted by heavy metals.
This is particularly true of mining operations which may have ground water flowing through them and exits into local streams and drinking water. Heavy metal contamination of natural water sources continues to be a problem in the mining communities long after mining has ceased. Due to the undesirability of heavy metal pollution in the water, much effort and expense is necessary to remediate these problems, often with less than adequate results.
Many problems exist with traditional heavy metal or water treatment remediation methods. As mentioned above, the chemistry of the metals involved is well known, so the various known processes are documented. One such method is metal hydroxide precipitation to remove heavy metals; however, this and many other conventional methods involve adding large quantities of chemicals to the waste stream which might contain quantities of contaminants at levels less than parts per thousand. These types of procedures can result in large quantities of metal-contaminated or metal-containing precipitate. As mentioned above, the disposal of the resultant metal-laden precipitate presents disposal problems of its own, particularly if the precipitate has the potential of later leaching of metals back into the environment.
Most of the known processes require complex and bulky equipment. These processes are expensive and sometimes result in less than adequate cleaning of the water. Conventional processes often result in a waste material that itself is hazardous and must be disposed of using expensive techniques which are the subject of further EPA regulation.
Thus, it would be a significant advancement in the art to provide improved methods and apparatus for cleaning water. More particularly, it would be a significant advancement in the art to provide such methods and apparatus which were capable of removing heavy metals from water. It would be an advancement in the art to provide such methods and apparatus which were capable of removing heavy metals from water without the use of chemical additives that produce large quantities of unstable chemical sludge. It would be a further advancement in the art to provide such methods and apparatus that operated using facilities significantly smaller than conventional water treatment facilities. It would be an advancement in the art to provide such methods and apparatus which were less costly to operate than conventional apparatus, and which were capable of producing waste products that were not themselves hazardous.
The methods and apparatus of the present invention have been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available water treatment methods and apparatus. To achieve the desired advantages mentioned above, and in accordance with the invention as embodied and broadly described herein in the preferred embodiment, methods and apparatus for removing heavy metals from water are disclosed herein.
In one aspect the present invention relates to a process for removing heavy metals from water. As discussed above, the removal of heavy metals from water is a major problem in maintaining and improving water quality. The process involves introducing or forming magnetite in water containing heavy metals such that the heavy metals are bound to the magnetite. Magnetite is an iron oxide with the formula Fe3O4. Magnetite occurs as a mineral and is a multi-valence oxide having both Fe(II) and Fe(III) in the same inverse spinel structure. It is found that magnetite is extraordinarily magnetic. Also, because of the inverse spinel structure of the mineral, it is possible for other metals to become incorporated into the magnetite crystal matrix. Metals can be incorporated chemically, or they can be absorbed onto the existing magnetite structure. In either event, these phenomena will be referred to herein as xe2x80x9cbindingxe2x80x9d or having the metal xe2x80x9cboundxe2x80x9d to the magnetite. These terms are used to incorporate both chemical binding and adsorption for purposes of describing the present invention. In addition, the resulting products will be referred to collectively as xe2x80x9cmagnetite.xe2x80x9d Compounds which include metals chemically bonded within the magnetite structure are sometimes referred to in the art as xe2x80x9cferrites.xe2x80x9d However, it will be understood, that for the purposes of this invention, such xe2x80x9cferritexe2x80x9d compounds fall within the scope of the term xe2x80x9cmagnetitexe2x80x9d as used herein.
The process of the present invention includes the steps of introducing magnetite to a quantity of water containing at least one heavy metal. One example of such water is mine drainage. Some of the experimental examples provided below use mine effluent from the Leadville mine drainage tunnel at Leadville, Colo. in order to demonstrate the present invention. Next, magnetite is mixed with the water, or formed in-situ, such that at least a portion of, and preferably most of, the heavy metal in the water is bound to the magnetite. Once the heavy metal is bound to the magnetite, the magnetite and heavy metal are removed by the application of a magnetic field. Since the magnetite is magnetic, it is easily separated from water by the application of a magnetic field in the presence of a magnetically active capturing matrix.
In most embodiments of the present invention the application of a magnetic field is accomplished by flowing the water through a solid magnetized matrix such that the magnetite magnetically binds to the solid matrix. The magnetized matrix may, for example, comprise steel wool. The steel wool can either be pre-magnetized, and therefore demonstrate xe2x80x9cremnant magnetism,xe2x80x9d or the steel wool can be placed under the influence of an external magnetic field generated by either an electromagnet or a permanent magnet.
In this configuration good results are achieved when the superficial velocity of the water through the matrix is in the range of from about 0.5 cm/sec to about 2.0 cm/sec. More preferably, the superficial velocity of water through the matrix is about 1 cm/sec. In order to adjust to larger or smaller inlet flow rates of water, the apparatus of the present invention is modular and scalable such that different flow rates can be handled while maintaining the superficial velocity within the preferred range.
Magnetite can be introduced to the process in several ways. The two main types of introduction of magnetite include the introduction of pre-formed magnetite to the water and the formation of magnetite in situ. The introduction of pre-formed magnetite may involve simply adding commercially available magnetite to the water to be treated. When using this process it is expected that the heavy metal removed will be metal adsorbed onto the magnetite.
Alternatively, the magnetite may be made in situ. In this process, effective quantities of Fe(II) and Fe(III), such as in the form of Fe(II) sulfate and Fe(III) sulfate, are added to the water. It is believed that magnetite is formed in a two stage process. First xe2x80x9cgreen rust,xe2x80x9d which is (Fe(II) oxy-hydroxides), are formed. This is followed by dehydration with and incorporation of Fe(III) to form magnetite. It is believed that in-situ formation of magnetite (ferrite) involves the incorporation of at least a portion of the heavy metal into the magnetite structure. Thus, the heavy metal is chemically bound within the magnetite inverse spinel structure.
A wide variety of heavy metals can be removed from water using the present invention. Examples of the heavy metals which can be removed from water include lead, manganese, cadmium, cobalt, mercury, nickel, and silver. In essence, the process is widely applicable to transition metals, actinides, and lanthanides.
Once the magnetite (ferrite) is formed and the heavy metals are bound to the magnetite, the resulting complex is flowed through a solid matrix. As mentioned above, the matrix may, for example, be steel wool. Also as mentioned above, the steel wool may display remnant magnetism, or it may be subject to an external magnetic field.
As the heavy metal-magnetite complex passes through the solid matrix, it is magnetically bound to the matrix. Thus, the heavy metal is removed from the aqueous system. Once the matrix becomes loaded with the bound magnetite it is a simple matter to remove and dispose of the bound magnetite. One such removal process is to reverse flow through the apparatus and increase the superficial velocity such that the magnetite is physically removed from the solid magnetic matrix. Generally a reverse flow of water, or more preferably an air-water mixture, is suitable. Flow rates somewhat higher than the initial flow rates thrugh the device are preferred. In particular, it is found that superficial velocities in the range of from about 3 cm/sec to about 10 cm/sec work well. This is in contrast to forward superficial velocities in the range of about 1.0 cm/sec.
The heavy metal bound magnetite is then collected and disposed of. It is found that, especially in the case of magnetite formed in situ, the heavy metal does not leach appreciably after disposal. Therefore, it is possible to use inexpensive disposal methods.
The present invention also relates to an apparatus for achieving the heavy metal remediation process described above. The apparatus can be constructed, for the most part of conventional components. In basic terms, the apparatus of the present invention includes a water conduit for introducing water containing heavy metals into the apparatus. The apparatus also includes a conduit for introducing a quantity of magnetite, or the components to synthesize magnetite in situ, into the apparatus. The water and the magnetite are mixed in a reaction chamber such that at least a portion of the heavy metals are bound to the magnetite. Next the mixture of water and heavy metal bound magnetite is introduced into a magnetic separator configured such that magnetite is removed from the water by application of a magnetic field. The separator preferably includes a solid matrix positioned within the separator and containing a remnant magnetization. The treated water is then conducted back out of the device by an outlet conduit.
As discussed above, the matrix within the separator may have remnant magnetism. Alternatively, an external magnet is provided to produce the necessary magnetic field gradients in the matrix material.
There are several advantages to the present invention over the conventional technology of heavy metal removal. The present invention provides a much smaller facility footprint, less chemical additives required and consequently less sludge produced than with conventional processes. Magnetic separation will even work on elements that are not intrinsically ferromagnetic or paramagnetic by the proper seeding of iron based compounds to the waste stream which act to scavenge the nonmagnetic contaminants.
The present invention demonstrates the several advantages of this technology. It has been found that remnant field separator performance on commercial magnetite at particle sizes greater than 5 xcexcm is more than adequate. Collection in and purging of the matrix can be controlled by superficial velocity and gas sparging. No magnetic field adjustment is required. Remnant field matrices appear to be magnetically stable and require minimal maintenance. Use of a graded matrix may effectively address a wider range of particle sizes if that is required.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.